An electric power steering apparatus which provides a steering system of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel (handle) 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the steering wheel 1, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command on the basis of a steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 by means of a voltage control value Vref obtained by performing compensation or the like to the calculated current command value. A steering angle sensor 14 is not indispensable and may not be provided. It is possible to obtain the steering angle from a rotational position sensor which is connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
In such an electric power steering apparatus, the control unit 30 mainly comprises a CPU (Central Processing Unit) (including an MPU (Micro Processor Unit) and an MCU (Micro Controller Unit)), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. The steering torque Th from the torque sensor 10 and the vehicle speed Vel from the vehicle speed sensor 12 are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 based on the steering torque Th and the vehicle speed Vel by using an assist map or the like. The calculated current command value Iref1 is added with a compensation signal CM for improving characteristics from a compensating section 34 at an adding section 32A. The current command value Iref2 after the addition is limited the maximum value thereof at a current limiting section 33. The current command value Irefm limited the maximum value is inputted into a subtracting section 32B, whereat a detected motor current value Im is subtracted from the current command value Irefm.
The subtraction result I (=Irefm−Im) in the subtracting section 32B is current-controlled at the current control section 35 such as a proportional-integral (PI) control and so on. The voltage control value Vref obtained by the current control is inputted into a PWM-control section 36, whereat a duty thereof is calculated. The motor 20 is PWM-driven by an inverter 37 with a PWM signal calculated the duty. The motor current value Im of the motor 20 is detected by a motor current detection means 38 and is inputted into the subtracting section 32B for the feedback. Further, a rotational sensor 21 such as a resolver is connected to the motor 20 and a motor angle θm is detected and is inputted.
The compensating section 34 adds a self-aligning torque (SAT) detected or estimated and an inertia compensation value 342 at an adding section 344. The addition result is further added with a convergence control value 341 at an adding section 345. The addition result is inputted into the adding section 32A serving as the compensation signal CM, thereby to improve the control characteristics.
In such a motor control unit, a vector control method that controls currents corresponding to respective axes (a d-axis current command value and a q-axis current command value) by using the vector since an angle relationship between the d-axis being a coordinate axis of a rotor of a three-phase brushless motor and the q-axis controlling strength of a magnetic field of the three-phase brushless motor is 90° and the q-axis and the d-axis are independent, is known.
An example of the vector control method is shown in FIG. 3. An angular speed calculating section 56 to calculate a motor angular speed ωm from a motor angle (a rotational angle) θ is provided, and the calculated motor angular speed ωm is inputted into a current command value calculating section 50 and a lead angle calculating section 57. The current command value calculating section 50 calculates a d-axis current command value Idref and a q-axis current command value Iqref of a dq-axis coordinate system of two axes based on the steering torque (torque command value) Th and the vehicle speed Vel, the d-axis current command value Idref and the q-axis current command value Iqref are inputted into a two-phase/three-phase converting section 51, and the two-phase/three-phase converting section 51 outputs three-phase current command value Iuref, Ivref and Iwref based on a motor angle θe being lead-angle corrected. That is, the current command value calculating section 50 calculates a current command value Iref based on the steering torque Th and the vehicle speed Vel, and calculates the d-axis current command value Idref and the q-axis current command value Iqref based on the calculated current command value Iref and the motor angular speed ωm. The d-axis current command value Idref is inputted into the lead angle calculating section 57, and the calculated lead angle θ0 is inputted into an adding section 58. The corrected motor angle θe is calculated by adding the lead angle θ0 to the motor angle θm, and the motor angle θe is inputted into the two-phase/three-phase converting section 51.
The three-phase current command values Iuref, Ivref and Iwref outputted from the two-phase/three-phase converting section 51, are respectively inputted into a subtracting section 52 (subtracting sections 52u, 52v and 52w), and deviations ΔIu, ΔIv and ΔIw which are subtracted respective phase currents Imu, Imv and Imw being detected at a motor current detecting section 55A from the current command values Iuref, Ivref and Iwref, are calculated at the subtracting sections 52. The deviations ΔIu, ΔIv and ΔIw are inputted into a PI-control section 53, and further the motor 20 is driving-controlled via a PWM-control section 54 and an inverter 55.
Although the three-phase currents are fed-back in the vector control method as shown in FIG. 3, a two-phase feedback type motor control unit that feedbacks the motor detecting current being performed a three-phase/two-phase conversion (U-phase, V-phase and W-phase currents→dq-axis currents), is also known (for example, Japanese Unexamined Patent Publication No. 2008-211908 A: Patent Document 1). In the unit of Patent Document 1, the method that obtains respective phase currents which have a desired torque based on a back-EMF measured-value between motor terminals and obtains dq-axis current command values by performing the three-phase/two-phase conversion, is disclosed. Further, in the unit of Patent Document 1, a torque ripple due to a strain component being contained in the back-EMF measured-value is compensated.